In an oxidizing medium, the ability of such parts to conserve good mechanical properties at high temperatures depends on providing the carbon with effective protection against oxidation. After being prepared, the composite material usually presents residual internal pores that give the ambient medium access to the core of the material.
Furthermore, in certain applications, the protection against oxidation must continue to be effective in the presence of moisture and/or catalysts for oxidizing carbon. This applies in particular to airplane brake disks made of C/C composite since they can be exposed to water present on runways and they can come into contact with catalysts for oxidizing carbon, e.g. provided by potassium formiates or acetates present in de-icing substances commonly used on runways.
Document US 2008/311301 describes a method of providing protection against oxidation for parts made of porous material containing carbon, and in particular brake disks made of C/C composite material, which method consists in applying a composition in an aqueous medium that comprises: a metallic phosphate in aqueous solution such as aluminum hydrogen-phosphate Al(H2PO4)3 or “monoaluminum phosphate” (MALP), a powder of boron or of a boron compound such as B2O3, titanium powder, and an alkali or alkaline earth element M that catalyzes carbon oxidation, in the presence of oxygen, so as to form a P—O—Ti-M association bonded by the boron oxide B2O3 and trapping the element M. Final heat treatment is performed at about 350° C. under air, and possibly at about 700° C. under a non-oxidizing atmosphere (N2). Such a composition is effective against catalytic oxidation of carbon. Nevertheless, the use of boron powder in an industrial environment can require special precautions to be put into place because of its toxicity. In addition, boron oxide B2O3 presents high sensitivity to moisture and vaporizes at temperatures below 900° C. In addition, the composition may be unstable and difficult to use, since boron and B2O2 react with monoaluminum phosphate.
Document US 2007/0026153 also describes a method of protecting parts made of carbon-containing composite material, and in particular brake disks made of C/C composite material. In a first step of the method, internal protection is put into place by impregnating with an aqueous solution of metallic phosphate, such as monoaluminum phosphate, and applying heat treatment at about 700° C. under nitrogen (N2). Thereafter, a liquid composition is applied that contains, in an aqueous medium, colloidal silica SiO2, a powder of “Pyrex®” type borosilicate glass, and powdered titanium diboride TiB2. In service and in an oxidizing medium, TiB2 oxidizes so as to form B2O3, thereby regenerating the B2O3 and conserving a borosilicate glass phase having healing capacity by passing into a pasty state as from 600° C. Nevertheless, the lifetime of the protection is relatively limited in the event of repeated exposure to a wet environment because of the disappearance of B2O3 and the consumption of TiB2. In addition, improving protection against catalytic oxidation of carbon may be desirable, in particular with respect to de-icing agents of the potassium formiate type, which can have a relatively strong tendency to become impregnated within the material because of their surface tension.